Gallium nitride-based (GaN-based) white light emitting diodes (LEDs) have high energy conversion efficiency, long life time, strong directionality of emitted light, low operating voltage, short warm-up time, and simple operating circuit. Also, GaN-based white LEDs have durability against external shocks and vibration in order to realize high-grade light systems having various kinds of packages. Thus, GaN-based white LEDs may change outdoorlight sources to solid-state lighting such as incandescent lamps, fluorescent lamps, and mercury lamps in the near future. To utilize GaN-based LEDs as a white light source that replaces outdoor fluorescent lamps and mercury lamps, the GaN-based LEDs should emit a high efficiency of output light at a low level of consumed power and also have excellent thermal stability. Lateral type GaN-based LEDs that are widely used have a relatively low manufacturing cost and simple manufacturing process. However, they are not adapted for use as an outdoor light source because they have a high applied current and a low light output. A vertical type LEDs may solve the limitations of the lateral type LED and be easily applied to a high performance LED having a high light output in a large area. Vertical type LEDs have many advantages compared to outdoor lateral type devices. Vertical type LEDs may have a low operating voltage and a high light output because they have a low current diffused resistor and obtain a uniform current spreading. Vertical type LEDs may have a long life time and a high light output that are significantly improved because heat is easily transfer to outside through a metal or semiconductor substrate having superior thermal conductivity. Vertical type LEDs may be widely used as a white light source for lighting because they have a maximum applied current of about three or four times that of lateral type LEDs. Research and development is being actively carried out on GaN-based vertical type LED for commercialization and improved performance by leading LED companies such as NICHIA CHEMICAL CO., LTD, JAPAN, PHILIPS LUMILEDS LIGHTING COMPANY, USA, and OSRAM, GERMANY and SEOUL SEMICONDUCTOR CO., LTD, SAMSUNG ELECTRO-MECHANICS CO., LTD, and LG INNOTEK CO., LTD, Korea.
The light output of a device may be significantly improved by an n-type semiconductor layer which is placed on the top layer on the device in the manufacturing of GaN-based vertical type LEDs. In the case where the semiconductor layer has a smooth surface, total reflection occurs at the interface between the atmosphere and the semiconductor layer by means of the refractive index difference between the semiconductor and the atmosphere (the refractive index of the n-type semiconductor layer is 2.4 or less, and the refractive index of the atmosphere is 1). The device may not have a highly extracted light output because light is emitted from an active layer—i.e., the light emitting layer does not emit to the outside. Thus, minimizing loss from light guiding on semiconductor inside is required by preventing total reflection by artificially changing the semiconductor surface.
For this, in the related art, a nano structure such as a silica (SiO2) nanosphere or polystyrene nanosphere is directly coated on a semiconductor layer to form a nano structure having a hemispherical shape on a surface of the semiconductor layer, and then the nano structure having the hemispherical shape is grown on the semiconductor layer by using a dry etching process to improve light extraction efficiency of the LED. However, in the above-described method according to the related art, when the nano structure is coated or etched, reproducibility may be deteriorated, and manufacturing costs may be increased. Thus, it may be difficult to manufacture a large-scale LED.